It has been known for more than 30 years that glasses doped with erbium ions can operate as lasers (see, e.g., E. Snitzer & R. F. Woodcock, "Yb.sup.3 +-Er.sup.3 + Glass Laser," Appl. Phys. Lett. 6, 45 (1965)). Early work on erbium glass lasers used silicate glasses and incorporated ytterbium ions as a sensitizer that would absorb pump energy and transfer it to the erbium ions. Within a few years, however, it was shown that phosphate glass was a significantly better material for this ytterbium sensitized erbium laser system (see, E. Snitzer, R. F. Woodcock & J. Segre, "Phosphate Glass Er.sup.3 +Laser," IEEE J. Quantum Electronics 4, 360, (1968)). Subsequent work with other glasses and crystals demonstrated that, because of its phonon energies, phosphate glass is a uniquely efficient host material for this laser system (see, e.g., V. P. Gapontsev et al., "Erbium Glass Lasers and Their Applications," Opt. Laser Technol., 189 (1982)).
A laser using ytterbium-sensitized erbium-doped phosphate glass as the gain medium can be pumped with different types of pump sources. Ytterbium in phosphate glass has a trivalent bonding state, and has a broad absorption peak stretching from 800 nm to 1100 nm, with a peak at 975 nm. Well established pump sources include InGaAs laser diodes generating wavelengths between 940 nm and 990 nm, and neodymium lasers generating wavelengths between 1040 nm and 1080 nm. U.S. Pat. No. 3,582,820 to Snitzer discusses intracavity pumping of such a laser with neodymium lasers. End pumping with a neodymium laser has been discussed in detail by D. Hanna, et al., in Optics Commun. 63, 417 (1987). A compact intracavity pumped erbium laser has been described by D. W. Anthon & T. J. Pier,in "Diode Pumped Erbium Glass Lasers," Solid State Lasers III, Gregory J. Quarles, Editor, Proc. SPIE 1627, 8-12 (1992). Pumping with laser diodes in the 940 nm to 990 nm region has been used in a side-pumped configuration by J. A. Hutchinson & T. H. Allik, in "Diode Array Pumped Er,Yb: Phosphate Glass Laser," Appl. Phys. Lett. 60, 1424-6 (1992), and in an end pumped geometry by P. Laporta et al., in "Diode Pumped CW Bulk Er:Yb:Glass Laser," Optics. Lett. 16, 1952 (1991).
Recent interest in erbium glass lasers comes from the desire to produce a suitable laser source for externally modulated CATV transmission systems. In a CATV system, analog optical signals are transmitted through optical fibers. Nd:YAG lasers operating at the wavelength of 1318 nm have been shown to be acceptable sources, and much of the experience with the CATV technology has been achieved using these devices. Nevertheless, it is highly desirable to provide a suitable laser source that operates at 1550 nm wavelength. This is because a typical fused silica optical fiber has the lowest attenuation around that wavelength. The low attenuation allows an optical signal at that wavelength to be transmitted over a longer distance. Because the gain of erbium doped glass covers a range of wavelengths centered around 1550 nm, there is currently strong interest in developing suitable erbium glass lasers to transmit CATV signals.
There are, however, numerous requirements for a laser used in such an application. For example, optimal operation of a CATV transmitter requires a laser with a relatively high optical power (typically greater than 100 mW into an optical fiber), very low relative intensity noise (RIN) (typically less than -160 dB/Hz) between 10 MHz and 1000 MHz, and an output spectrum that is consistent with minimized noises associated with dispersion, stimulated Brillouin scattering (SBS) and phase noise.
One way to minimize stimulated Brillouin scattering (SBS), which is one of the main sources of noises in signal transmission through an optical fiber, is to use a laser source that operates in multiple modes, where the spacing between those modes is more than twice the highest frequency in the system. With such mode spacing, the noise associated with mode beating does not appear in the signal band.
In a multimode laser, the output power is distributed in several spectral modes instead of being concentrated in one single mode, which is useful with respect to the suppression of SBS. The mode spacing in a typical neodymium or erbium laser is large with respect to the 100 MHz Brillouin linewidth. As a result, the individual modes act independently with respect to the onset of SBS (see, e.g., Y. Aoki and K. Tajima, "Stimulated Brillouin scattering in a long single-mode fiber excited with a multimode pump laser," J. Optical Society B 5(2), 358-363 (1988)).
The threshold for SBS is determined by the power carried in the most intense laser mode. By spreading the power over multiple spectral modes, the power in each mode can be kept below the SBS threshold, thereby suppressing noise caused by SBS. In this way, the power that can be transmitted through the fiber without significant SBS-related noise is increased. This is important for long distance analog transmission systems such as CATV, where signal-to-noise considerations dictate the use of relatively high power laser sources.
Besides the advantage of suppressed SBS, a laser source that operates in multiple spectral modes has the further advantage of improved stability of operation. Multi-longitudinal mode operation is often more stable than single frequency operation. Single frequency lasers are particularly unstable in the vicinity of a mode hop, and external perturbations can cause mode hops that drastically change the properties of a single frequency laser. The effect of mode hoping on the quality of the output of a diode-pumped laser can be seen if the output power of the laser is plotted as a function of the current flowing into the pump diode. In particular, the output power versus diode current (L-I) curve of a single-mode laser often exhibits steps or dips near mode hops. Multimode lasers are less prone to sudden mode changes and, as a result, produce relatively smooth L-I curves.
The number of modes in the output of a laser can sometimes be increased by broadening the bandwidth of the laser, which can further enhance suppression of SBS. This approach, however, is not always applicable because the acceptable bandwidth of a laser is often dictated by the application in which the laser is used. For example, in the CATV application, the upper limit on the useful laser bandwidth is determined by dispersion in the optical fiber, and is inversely proportional to the signal bandwidth, the fiber length and the group velocity dispersion. The dispersion of a typical single mode fiber at the wavelength of 1318 nm, where the optical fiber has the lowest dispersion, is less than 2 ps/(nm km). At 1550 nm, the dispersion is approximately 10 times that value. At the wavelength of 1318 nm, a typical allowable bandwidth is less than 20 nm, and probably closer to five (5) nm. A 20 nm bandwidth is too large for CATV systems as can be seen from the problems reported by M. Nazarathy, J. Berger et al. in "Progress in Externally Modulated AM CATV Transmission Systems," J. Lightwave Technology 11(1), 82-105 (1993), where "dual line" (1318+1338 nm) Nd:YAG lasers were used in CATV systems. A five (5) nm dispersion limit at 1318 nm corresponds roughly to a bandwidth of 800 GHz. Such a bandwidth is much larger than the cavity mode spacing of a typical Nd:YAG laser, which is on the order of 10 GHz. It is therefore possible to operate a Nd:YAG laser with a high number of output modes within such a bandwidth even though not all available cavity modes will be excited.
On the other hand, the dispersion-limited bandwidth at 1550 nm wavelength is much narrower than that at 1318 nm. Because the dispersion at 1550 nm is about ten times higher than that at 1318 nm, a five (5) nm dispersion limit at 1318 nm would become 0.5 nm at 1550 nm, which corresponds to a bandwidth about 60 GHz. The cavity mode spacing of a typical erbium laser is in the six (6) to ten (10) GHz range, which means there will be at most six to ten available cavity modes in the desired bandwidth. Such a small number of available cavity modes makes it extremely difficult to achieve multimode operation in an erbium laser and results in the laser having a strong tendency to operate in a single mode when the bandwidth is narrowed to about 60 GHz.
Some other difficulties encountered in developing a suitable erbium laser for CATV application are related to the relatively poor thermal and physical properties of erbium glass. An erbium laser for CATV application has to be able to provide an output power on the order of 150 mW. Due to the poor thermal conductivity of phosphate glass--the host material for the erbium ions--operating the erbium laser at such a high power (e.g., 150 mW) can produce a significant heat load on the gain medium. The heating problem is often exacerbated by the fact the gain medium in a erbium laser, which is a three level laser, is often short in length in order to minimize reabsorption losses. In such a case, the heat load in the erbium-doped phosphate glass is concentrated in a relatively small volume and can cause severe damage to the glass, such as local surface melting and fracture. The heat load in the glass can also cause thermal lensing, which degrades the quality and power of the laser output.